1. Field of the Invention
This invention relates to a radio frequency choke and, more particularly, to a choke for separating or combining an AC power signal and an RF signal.
2. Prior Art
Commonly, cable television (CATV) distribution systems use broadband systems (5 to 1000 MHz) to carry various channel information to a subscriber and subscriber information back to the CATV provider. In hybrid fiber coax (HFC), a prevailing and commonly implemented architecture, the RF signal is transmitted from the CATV provider through optical fiber to an optical node/receiver. The RF signal is then transmitted through coaxial cable to the subscriber. The RF signal originating from the subscriber is transmitted through the coax plan (that section of the CATV system that transmits the RF signal over coaxial cable) to the optical node where it is transmitted through optical fiber to the CATV provider. Some CATV systems utilize coax plan only, without the use of any optical fiber in the system.
FIG. 1 shows a hybrid fiber coax (HFC) CATV system 100. The CATV provider, represented by head end 101, includes optical transmitters 102 for transmission of the RF signal through optical fiber 103 to optical node 104. Optical node 104 receives the RF signal from head end 101 and an AC power signal from AC power supply 105. Optical node 104 combines the RF signal and the AC power signal and transmits them onto coaxial cable 108. Various devices are attached to coaxial cable 108, including repeaters 106 and subscriber's receiver 107. The section of HFC system 100 that operates on coaxial cable, i.e. optical node 104, repeaters 106 and receiver 107, is collectively referred to as a coax plan 109.
The data for the various channels is typically transmitted at frequencies of between 50 and 1000 MHz while subscriber information sent to the CATV provider is typically transmitted at frequencies of between 5 and 40 MHz. In addition to the RF signals, a 60 Hz single phase power signal is sent from optical node 104 over the same coaxial cable in order to power various devices (e.g., RF amplifiers) that are attached to the coaxial cable at various points in the cable system. Typically, the 60 Hz AC power signal is at 60 volts and has a current magnitude of up to 15 A. In contrast, the broadband RF signal typically has a peak voltage of 0.3 V.
RF chokes are used in the various devices, such as repeaters 106, attached to coaxial cables line 108 and at the subscriber's receiving station 10)7. The RF choke separates the RF signal from the single-phase AC power signal by presenting a low impedance to the 60 Hz AC power signal while presenting a high impedance to the 5-1000 MHz RF signal, thereby shunting the AC power signal through the RF choke while blocking passage of the RF signal. Additionally, an RF choke can be used to combine an RF signal with an AC power signal, as is required in optical node 104 and in repeaters 106. The ideal RF choke would pass all of the AC power signal through the choke while blocking all of the broadband RF signal.
A typical device present on coaxial plan 109 is repeater 106. Repeaters 106 are inserted at various locations along coaxial cable line 108 and perform the function of insuring that the RF signal that arrives at the subscriber station is sufficiently intense to provide good television reception. At repeater 106, an RF choke is used to separate the RF signal from the AC power signal. The AC power signal is used to power an amplifier that amplifies the RF signal. The amplified RF signal and the AC power signal are then recombined, again using an RF choke, for transmission to the next device on coaxial cable 108. It is important, especially in coax plans 109 having multiple devices at various locations along coaxial cable 108, that each device have a relatively flat frequency response to minimize distortion in the RF signal. In separating the AC power signal from the RF signal, therefore, the RF choke should not leak a significant amount of the RF signal into the AC power signal, thereby causing significant frequency dependent loss of RF signal.
Typically, an RF choke comprises a number of turns of magnetic wire around a magnetic core to form coils. A resistor may also be inserted across a portion of the magnetic wire coils. There are essentially four, often conflicting, considerations concerning an RF choke. One such consideration is the frequency response of the RF choke. The self-capacitance between the turns of wire in the choke, combined with the inductance of the choke, produces various LC resonances that often fall within the broadband RF signal frequencies. The result of these resonances is a loss of RF signal power through the RF choke at those resonances. The cumulative effects of all of the various RF signal power losses in the transmission can attenuate the RF signal at that resonant frequency, especially where the same or similar components having similar frequency responses are used in various devices throughout the cable. A shunt resistor may mitigate this effect by destroying the quality factor Q of the LC resonance. However, the presence of the shunt resistor also reduces the RF signal impedance to ground, thereby increasing RF signal loss. Additionally, epoxies used to hold the turns of wiring on the magnetic core also operate to increase the self-capacitance between wires by increasing the dielectric constant between wires, thereby worsening the frequency response.
A second consideration concerns the frequency response and return loss at low frequencies. If the RF choke has too much inductance, resonances are created at high RF frequencies because of the Inherent self-capacitance. However, too little inductance results in excessive insertion loss and return loss at low RF frequencies.
Another consideration is "hum" modulation. Hum modulation refers to the distortion of the RF signal that results from saturation of the RF choke because of the high current of the AC power signal. With high currents, core materials are likely to approach magnetic saturation, thereby presenting the RF signal with an impedance that varies with the frequency of changes in the AC power signal.
In addition, the RF choke must be able to carry up to 15 A of current associated with the AC power signal. Typically, chokes of this type use 18 to 20 gauge wiring, which is not rated sufficiently high to carry 15 A currents. For example, 18 gauge wire is sufficient to carry about 10 A of current. In addition, the RF choke must be able to efficiently dissipate heat.
Therefore, there is a need for an RF choke with a flat frequency response in the 5-1000 MHz range, good return loss, good hum modulation characteristics, and high current carrying capacity.